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Stray-Light Contamination and Spatial Deconvolution of Slit-Spectrograph Observations

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Stray-Light Contamination and Spatial Deconvolution of Slit-Spectrograph Observations A&A 535, A129 (2011) Astronomy DOI: 10.1051/0004-6361/201015702 & c ESO 2011 Astrophysics Stray-light contamination and spatial deconvolution of slit-spectrograph observations C. Beck1,2, R. Rezaei3, and D. Fabbian1,2 1 Instituto de Astrofísica de Canarias (CSIC), Vía Láctea, 38205 La Laguna, Tenerife, Spain e-mail: [cbeck,damian]@iac.es 2 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain 3 Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: [email protected] Received 7 September 2010 / Accepted 9 September 2011 ABSTRACT Context. Stray light caused by scattering on optical surfaces and in the Earth’s atmosphere degrades the spatial resolution of ob- servations. Whereas post-facto reconstruction techniques are common for 2D imaging and spectroscopy, similar options for slit- spectrograph data are rarely applied. Aims. We study the contribution of stray light to the two channels of the POlarimetric LIttrow Spectrograph (POLIS) at 396 nm and 630 nm as an example of a slit-spectrograph instrument. We test the performance of different methods of stray-light correction and spatial deconvolution to improve the spatial resolution post-facto. Methods. We model the stray light as having two components: a spectrally dispersed component and a “parasitic” component of spectrally undispersed light caused by scattering inside the spectrograph. We used several measurements to estimate the two contribu- tions: a) observations with a (partly) blocked field of view (FOV); b) a convolution of the FTS spectral atlas; c) imaging of the spider mounting in the pupil plane; d) umbral profiles; and e) spurious polarization signal in telluric spectral lines. The measurements with a partly blocked FOV in the focal plane allowed us to estimate the spatial point spread function (PSF) of POLIS and the main spec- trograph of the German Vacuum Tower Telescope (VTT). We then used the obtained PSF for a deconvolution of both spectroscopic and spectropolarimetric data and investigated the effect on the spectra. Results. The parasitic contribution can be directly and accurately determined for POLIS, amounting to about 5% (0.3%) of the (con- tinuum) intensity at 396 nm (630 nm). The spectrally dispersed stray light is less accessible because of its many contributing sources. We estimate a lower limit of about 10% across the full FOV for the dispersed stray light from umbral profiles. In quiet Sun regions, the stray-light level from the close surroundings (d < 2) of a given spatial point is about 20%. The stray light reduces to below 2% at a distance of 20 from a lit area for both POLIS and the main spectrograph. The spatial deconvolution using the PSF obtained improves the spatial resolution and increases the contrast, with a minor amplification of noise. Conclusions. A two-component model of the stray-light contributions seems to be sufficient for a basic correction of observed spectra. The instrumental PSF obtained can be used to model the off-limb stray light, to determine the stray-light contamination accurately for observation targets with large spatial intensity gradients such as sunspots, and also to improve the spatial resolution of observations post-facto. Key words. methods: data analysis – line: profiles – Sun: chromosphere 1. Introduction treatment of this contamination by using a separately provided stray-light profile (see also, e.g., Orozco Suárez et al. 2007a,b). The importance of stray-light contributions to observed spectra For analysis techniques that make direct use of the observed and images was realized very early on and continues to be a spectra or of broad-band images, the stray light has to be dealt problem today (see, e.g., Henoux 1969; Mattig 1971; Martinez with in advance. This problem had to be tackled in the con- Pillet 1992; Chae et al. 1998; Wedemeyer-Böhm 2008; DeForest text of the accurate determination of sunspot intensities to de- et al. 2009; Mathew et al. 2009). The intrinsically limited opti- rive their temperature at continuum forming layers (Kneer & cal quality of the reflective surfaces of mirrors or of the glass of Mattig 1968; Maltby & Mykland 1969; Maltby 1970; Mattig lenses leads to scattering of photons in the light path. In addition, 1971; Tritschler & Schmidt 2002) or in the determination of scattering during the passage through the Earth’s atmosphere by, the continuum contrast of solar granulation (Mathew et al. 2009; for instance, dust particles spreads the light from the full solar Wedemeyer-Böhm & Rouppe van der Voort 2009). It is also im- disk to any point of an observed restricted field of view (FOV). portant in studies of solar chemical abundances that seek to de- Depending on the wavelength and the target of the observations, rive consistent results from different spectral lines (Asplund et al. the stray light can amount to a significant fraction of the ob- 2009; Fabbian et al. 2010). served intensity and can have a strong impact on the final results The stray-light contribution is often decomposed in differ- of the data analysis. Therefore, many inversion codes that deter- ent sources: 1. the atmospheric stray light caused by large-scale mine physical quantities from observed spectra, such as the SIR scattering in the Earth’s atmosphere, with a slow temporal evo- code (Ruiz Cobo & del Toro Iniesta 1992), include an explicit lution because of the change of air-mass in the light path during Article published by EDP Sciences A129, page 1 of 13 A&A 535, A129 (2011) the day; 2. the “blurring”, i.e., the rapidly changing amount of case of ground-based observations. For intrinsically 2D observa- stray light caused by the fluctuations of the refractive index in the tions, whether broad-band imaging or narrow-band spectroscopy Earth’s atmosphere (which are nowadays commonly referred to or spectropolarimetry with suitable 2D instruments (see, e.g., as “seeing”); and 3. instrumental stray light, i.e., scattering in the Tritschler et al. 2002; Mikurda et al. 2006; Cavallini 2006; telescope/instrument optics or (static) stray-light effects of the Puschmann et al. 2006; Scharmer et al. 2008; Beck et al. 2010), telescope/instrument caused by the geometry of the finite aper- the instantaneous PSF can be estimated using series of rapid ture and the possible presence of (central) obscurations in the short-exposed images. Two different approaches, speckle imag- light path (Zwaan 1965; Staveland 1970; Mattig 1983; Martinez ing (e.g., von der Lühe 1993) and blind deconvolution (e.g., Pillet 1992; Wedemeyer-Böhm 2008). For space-based observa- van Noort et al. 2005, and references therein), then allow a post- tions, naturally, the first two points are absent. facto reconstruction of the most probable original solar image The stray-light contamination is quantified in the “spread that lies behind the observed image series. The reconstruction function”, whose large-scale and small-scale contributions are algorithms are, however, only capable of recovering the instan- usually approximated as analytical functions of varying shapes, taneous PSF within some limits and are also partially blind to e.g., Gaussian or Lorentzian, or with combinations of a number the static parts of the PSF (Scharmer et al. 2010). of them (Mattig 1971; Wedemeyer-Böhm 2008; DeForest et al. For slit-spectrograph observations, at any given moment of 2009; Mathew et al. 2009). For describing the purely instrumen- time, only a 1D slice of the solar surface is available, whereas tal effects without temporal dependence and in night-time as- the PSF is a 2D function that therefore also introduces stray tronomy, the label “spatial point spread function” (PSF) is com- light from locations that are not covered by the slit in that mo- monly used because the function describes the spatial shape that ment. Thus, even if both the static and the time-variant parts the light from a point source (as a star can be approximated to of the PSF are known, it is not straightforward to use this in- be) would attain after passing through the optical system. formation to improve the spatial resolution of slit-spectrograph To determine the exact shape of the spread function, either data post-facto. Keller & Johannesson (1995)andSütterlin & theoretical calculations of the optical systems or observations Wiehr (2000) introduced a method similar to a speckle deconvo- with an only partly illuminated FOV are used. The theoretical lution, requiring a fast series of slit-spectra. In our contribution, calculations are usually limited to the time-invariant effects of we tested the application of a post-facto correction of “standard” the optics, while for the varying contributions from, e.g., the slit-spectrograph data for the static part of the instrumental PSF seeing only some statistically averaged effects may be assumed. as obtained from a dedicated set of measurements. For determining the spread function in the solar case, suitable The stray-light contribution becomes significant when the observations can be obtained in different ways (Mattig 1971; light level in observations is low, as for observations of sunspots, Briand et al. 2006; Wedemeyer-Böhm 2008; DeForest et al. near the solar limb, or for spectroscopy of very deep spec- 2009), namely by planetary transits in front of the Sun, by ob- tral lines such as Hα or Ca ii H. For a recent study of the servations near and off the solar limb, or by artificial occulters in center-to-limb variation (CLV) of Ca ii H spectra taken with the the light path. Planetary transits are actually the best suited be- POlarimetric LIttrow Spectrograph (POLIS; Beck et al. 2005) cause they provide the perfect reference of an intrinsically zero at the German Vacuum Tower Telescope (VTT, Schröter et al. light level, but the diameters of the planets are usually smaller 1985), we thus had to investigate the stray-light contamination than the extent of the PSF and thus do not allow to determine the in detail.
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